/ 


No.  1008 


DESCRIPTION  AND  INSTRUCTIONS 

FOR  USE  OF 

WELDON  RANGE  FINDER 


(ONE  PLATE) 


REVISED  MARCH  1,  1906 
REVISED  FEBRUARY  1,  1909 
REVISED  SEPTEMBER  2,  1911 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1917 


A-  J. 


G'G'S 


oo 

CM 


War  Department, 

Office  of  the  Chief  of  Ordnance, 

Washington,  September  2,  1911. 

This  manual  is  published  for  the  information  and  government  of  the  Regular 
Army  and  Organized  Militia  of  the  United  States. 

By  order  of  the  Secretary  of  War  : 

William  Crozier, 

Brigadier  General,  Chief  of  Ordnance. 

(3) 


104720—17 


. 


TABLE  OF  CONTENTS. 


Page. 


Description- 
Construction. 

Use r____ 

Observations 

Instructions-  in  range  finding 10 

Range  finding 12 


(5) 


CD  00  00 


, 

. 

■ 


THE  WELDON  RANGE  FiNDER 


1.  The  Weldon  range  finder  is  a small  hand  instrument  used  in 
measuring  ranges.  The  range  finder  consists  of  a metal  case  apd  a 
cover. 

2.  Inside  the  case  are  mounted  three  prisms,  a needle,  and  a level. 
(Fig.  5.)  Between  the  first  and  second  prisms,  and  between  the  sec- 
ond and  third  prisms,  assembled  one  above  the  other,  a window  slot 
is  made.  By  this  arrangement,  when  the  instrument  is  opened  out 
and  the  protecting  cover  used  as  a handle,  the  three  prisms  can  be 
brought  into  position  before  the  eye  of  the  observer,  and  any  of 
them  can  be  used  without  necessity  for  the  observer  taking  his  eye 
off  the  object  he  is  observing. 

3.  Screwed  into  a seat  in  the  case  is  a magnetic  needle,  which  per- 
mits the  instrument  to  be  used  as  a compass.  The  metal  cover, 
which  forms  the  handle,  has  four  notches  cut  in  it.  These  serve  to 
support  a pencil  in  such  position  that  when  the  needle  points  due 
north  the  pencil  will  point  E.  and  W.,  NE.  and  SW.,  or  NW.  and 
SE.,  as  the  pencil  is  laid  either  transversely  or  diagonally  in  the 
notches.  (Fig.  5.) 

The  level,  fixed  below  the  third  prism,  is  used  to  assist  the  observer 
in  keeping  the  instrument  upright  when  finding  a range. 

Pasted  on  the  inside  of  the  cover  is  a sheet  of  memoranda  giving 
the  multiples  to  be  used  with  the  different  prisms. 

4.  With  each  Weldon  range  finder  a steel  60-foot  tape  is  issued  to 
each  company  of  infantry  and  engineers,  and  a 100-foot  tape  to 
mobile  artillery.  The  cases  of  tapes  and  instruments  are  finished 
with  olive-drab  enamel.  A leather  case  and  strap  is  issued  with  each 
range  finder,  in  which  it  should  habitually  be  kept  when  not  in  use.  • 

5.  The  range  finder  should  be  handled  with  due  care.  When 
necessary  to  clean  the  prisms,  use  a soft  cloth  to  remove  all  dirt. 
Dust  and  grit  should  be  removed  by  blowing  or  careful  wiping,  so  as 
not  to  scratch  the  prisms.  No  moisture  other  than  that  deposited 
from  the  breath  will  be  needed.  The  prisms  should  be  kept  per- 
fectly clean  for  observing.  Under  no  circumstances  should  the 
prisms  be  removed  from  their  seats  except  by  a skilled  workman  at 
shops  equipped  for  instrument  work. 

(7) 


8 


6.  The  steel  tape  is  graduated  on  one  side  in  feet,  tenths  of  feet, 
and  hundredths  of  feet,  and  on  the  other  side  in  yards.  It  is  an 
accurate  and  sensitive  instrument,  and  must  be  handled  with  skill 
and  care.  Avoid  jerking,  bending,  or  twisting  it.  If  sand,  grit,  or 
dirt  gets  on  it,  clean  it  before  winding.  Also  in  wet  weather,  or  in 
damp  localities,  keep  the  entire  length  of  the  tape  well  oiled  to 
prevent  rusting. 

I.  CONSTRUCTION. 

7.  The  instrument  consists  of  three  prisms  securely  fixed  in  metal 
frames.  Each  prism  reflects  at  a different  angle.  These  angles  are 
90\88°  51'  15",  and  74°  53'  15".1 

8.  For  convenience  of  reference,  the  three  prisms  are  called,  re- 
spectively, first,  second,  and  third. 

The  first  is  90° ; 

The  second  is  88°  51'  15"; 

The  third  is  74°  53'  15" ; 

and  this  is  the  order  in  which  they  will  most  frequently  be  used. 

The  angles  of  the  prisms  are  so  calculated  that  when  the  first  and 
second  prisms  are  used  together  in  the  manner  explained  below  the 
range  is  equal  to  50  times  the  base;  that  when  the  second  and  third 
prisms  are  used  together  the  range  is  equal  to  4 times  the  base ; and 
that  when  the  second  and  third  prisms  and  then  the  first  and  second 
prisms  are  used  together  (see  Problem  2)  the  range  is  equal  to  200 
times  the  base;  that  when  the  second  prism  is  used  alone  the  range 
is  25  times  the  base. 

II.  USE. 

9.  In  using  the  range  finder,  the  instrument  is  opened  out.  There 
are  two  positions  in  which  it  can  be  held : 

(a)  As  shown  in  figure  5,  with  the  apex  of  the  prism  toward  the 
eye  of  the  observer.  The  reflection  of  an  object  to  the  right  of  the 
observer  is  to  be  sought  for  in  that  side  of  the  prism  which  is  farthest 
from  the  object,  and  in  the  section  of  that  side  which  is  farthest 
from  the  apex  angle.  Thus,  to  reflect  0 , the  observer  would  look 
for  it  in  the  prism  at  (9',  figure  5.  This  method  is  preferable,  and 
should  be  used  when  the  target  is  to  right  of  observer. 

(&)  It  can  be  held  as  shown  in  figure  6,  with  the  apex  of  the  prism 
toward  the  object.  The  reflection  of  0 is  in  this  case  to  be  sought 
in  the  Corner  nearest  the  apex  of  the  prism.  This  method  of  holding 
will  be  used  when  the  target  is  to  the  left  of  observer.  , 

1 The  first  and  second  angles,  viz,  90°  and  88°  51'  15",  are  the  angles  at  the  base  of 
a right-angled  triangle  of  which  the  base  is  to  the  perpendicular  as  1 : 50.  The  comple- 
ment of  the  second,  viz,  91°  8'  45",  and  the  third  are  the  angles  of  an  obtuse-angled 
triangle,  of  which  the  base  is  to  the  side  subtending  the  angle  74°  53'  15"  as  1 : 4. 


9 


OBSERVATIONS. 

10.  In  the  first  or  right-angled  prism  the  three  corners  are  avail- 
able for  observation,  and  therefore  the  whole  surface  is  open.  In  a 
prism  reflecting  at  an  angle  not  a right  angle,  one  corner  is  not  avail- 
able for  observation ; in  the  second  and  third  prisms,  these  corners  are 
covered  or  cut  off. 

11.  To  obtain  a general  idea  of  the  method  of  using  the  range 
finder,  the  observer  stands  so  that  his  right  shoulder  points  to  the 
target — the  distance  of  which  he  is  about  to  find. 

Holding  the  instrument  by  the  cover,  he  looks  with  his  right  eye 
straight  in  the  top  prism  of  the  three,  and  in  that  quarter:  of  it  which 
is  nearest  to  the  figure  1 on  the  frame,  and  therein  the  object  sought 
will  be  seen  reflected. 

A little  practice  may  at  first  be  required  to  readily  discern  in  the 
prism  the  object  sought,  but  by  holding  the  instrument  close  to  the 
eye  a large  field  of  view  is  obtained,  and  a very  slight  movement 
of  the  head  to  the  right  or  left  is  all  that  is  required  to  bring  a fresh 
field  into  view.  In  observing,  several  images  may  be  seen.  Rays 
of  light  forming  the  desired  image  suffer  two  internal  reflections. 
If  the  range  finder  be  held  in  a vertical  position  and  rotated  about  a 
vertical  axis,  the  desired  image  will  remain  stationary,  whereas  the 
other  images  rotate  with  the  range  finder. 

The  image  of  the  target  must  now  be  made  to  coincide  with  some 
object  immediately  in  front  of  the  observer^  known  as  the  aiming 
point,  and  to  effect  this  he  must  look  into  and  also  over  the  prism  at 
one  and  the  same  time. 

Alignment  can  be  effected  by  the  observer  moving  to  his  right,  or 
left,  or  backward,  or  forward. 

Having  made  the  required  coincidence  of  the  image  of  the  target, 
with  the  aiming  point,  the  observer  has  formed  a right  angle  at  the 
prism  between  the  line  to  the  aiming  point  and  the  line  to  the  target. 

12.  The  observer  now  looks  in  the  second  prism,  and  sees  that  the 
image  of  the  target  and  the  aiming  point  do  not  coincide,  for  the 
target  is  seen  to  the  right  of  the  aiming  point  in  the  second  prism. 
To  obtain  the  range  of  the  target,  the  observer  must  make  the  image 
and  object  again  coincide  with  the  second  prism,  and  to  effect  this, 
he  marks  the  spot  upon  which  he  stands,  and  'moves  backward — 
keeping  the  aiming  point  and  his  first  position  in  alignment.  Having 
gone  some  distance,  he  again  looks  in  the  second  prism,  and  if  the 
image  of  the  target  still  falls  to  the  right  of  the  aiming  point,  it 
shows  that  he  has  not  gone  far  enough.  If,  however,  the  target  falls 
to  the  left  of  the  aiming  point,  it  proves  that  he  has  gone  too  far,  and 
he  must  retrace  his  steps  to  a position  where  the  images  coincide.  The 
range  of  the  target  is  equal  to  the  distance  from  this  second  position 


10 


(found  by  the  second  prism)  to  the  first  position  (found  by  the  first 
prism)  multiplied  by  50,  i.  e.,  if  20  yards  separate  the  two  positions, 
the  range  is  1,000  yards ; if  50  yards,  the  range  is  2,500  yards. 

13.  To  measure  an  angle  correctly,  the  image  of  the  target  should 
be  kept  upright,  and  the  image  of  the  horizon  as  level  as  it  is  natu- 
rally ; for  if  the  instrument  be  held  so  that  the  image  of  the  ground 
appears  to  slope  when  it  is  really  level,  or  made  to  appear  level  when 
it  slopes,  the  required  angle  will  not  be  measured.  In  observing  the 
bubble  of  the  small  spirit  level  should  be  first  centered,  and  the 
range  finder  rotated  away  from  or  toward  the  eye,  always  keeping 
the  bubble  centered, 'until  the  image  of  the  horizon  appears  at  it  is 
naturally,  or  until  vertical  objects  appear  vertical. 

14.  Figure  7 (using  prism  No.  1)  shows  how  alignment  or  coinci- 
dence of  the  target  is  made  with  the  aiming  point.  Thus,  if  an 
object,  (9,  figure  7,  appears  to  the  observer’s  left  of  an  object,  Z>,  in 
front  of  him,  O can  be  brought  immediately  under  D by  moving 
straight  forward  if  D is  distant,  or  to  the  right  or  diagonally  to  the 
front  if  D is  nearer.  If  (9,  figure  7,  appears  to  the  observer’s  right  of 
an  object,  say  G in  front,  he  must  step  back,  or  more  to  the  left,  until 
alignment  has  been  effected. 

15.  Any  glare  from  the  sun  on  the  instrument  can  be  avoided  by 
shading  the  instrument  with  the  unoccupied  hand. 

III.  INSTRUCTION  IN  RANGE  FINDING. 

16.  For  instruction  in  range  finding,  a pole  O (figure  1),  should  be 
set  up  at  a known  distance^  say  300  yards,  from  the  point  X. 

0 is  called  the  target. 

In  a line  at  right  angles  to  the  target  line  XO , but  at  a greater 
distance  than  target  O , let  another  pole  Z>,  be  set  up. 

D is  called  the  aiming  point. 

The  point  X , where  the  target  O forms  a true  right  angle  with  the 
aiming  point  Z>,  should  be  permanently  marked  by  a pole  or  picket 
k not  higher  than  4J  feet  from  the  ground  to  suit  observers  of  different 
heights,  so  that  the  instrument  can  be  conveniently  rested  on  it  and 
the  appearance  of  a correct  angle  noted. 

One-fiftieth  of  the  distance  of  O from  X (in  this  case,  6 yards) 
should  be  measured  from  J on  the  line  DX  prolonged,  and  the  point 
A so  found  should  be  marked  by  a short  line 1 cut  in  the  ground  across 
the  line  of  sight.  At  this  point  the  reflection  of  O in  the  second 
prism  will  coincide  with  the  aiming  point  D. 

17.  The  learner  having  been  taught  how  to  hold  the  instrument, 
and  to  observe  different  objects  to  the  right  or  left  of  him,  should  be 

1 The  extremities  of  this  short  line  might  be  permanently  marked  by  two  small  pegs, 
so  that  the  observer  may  have  nothing  but  the  instrument  to  guide  him  in  finding  the 
point  A. 


11 


taken  to  the  point  X , figure  1,  and  shown  how  the  pole  at  the  target 
coincides  with  the  pole  at  the  aiming  point  when  the  instrument  is 
held  properly,  and  what  the  effect  is  when  the  object  is  not  reflected 
upright.  He  will  then  be  provided  with  a picket  and  told  to  find  a 
right  angle  at  some  other  spot. 

18.  When  proficient  in  finding  the  required  angle,  the  learner  will 
again  be  taken  to  the  point  X,  figure  1,  and,  using  the  second  prism, 
he  will  note  how  the  target  0 is  reflected  to  his  right  toward  O' . He 
will  then  be  directed  to  find  the  point  A 
by  retiring  in  line  with  the  picket  X 
and  the  aiming  point  Z>,  and  noting 
that,  as  he  approaches  A,  the  image  of 
O approaches  I). 

19.  Then,  using  the  tape  provided,  he 
will  measure  the  base  AX.  If  the  tape 
used  is  20  yards  (60  feet)  long,  each 
such  length,  using  the  first  and  second 
prisms  (multiplier  50),  is  equivalent  to 
1,000  yards  of  range. 

20.  When  the  learner  is  proficient  in 
this  preliminary  practice,  he  may  be 
directed  to  take  short  ranges,  which  are 
known,  and  the  cause  of  any  error 
pointed  out. 

21.  In  using  the  second  prism  the  ob- 
server should  be  especially  cautioned  to 
fix  his  attention  on  the  far-off  aiming 
point,  and  not  on  any  intermediate 
point,  which  serves  to  guide  the  eye  in 
a straight  line  to  the  aiming  point. 

When  the  observer  comes  to  take 
long  ranges,  he  will  note  that  the  re- 
flection of  the  target  approaches  the 
aiming  point  more  slowly  than  it  did 
when  taking  short  ranges;  and  as,  after 
repeatedly  observing  the  object  on  one 
side  of  the  aiming  point,  the  eye  is  apt 
to  be  satisfied  with  what  is  in  reality 
only  an  approximate  coincidence,  he  should  retire,  or  advance,  until 
the  target  appears  on  the  other  side  of  the  aiming  point,  and  then 
proceed  to  take  off  what  is  necessary. 

For  extremely  long  ranges  it  might  be  safer  to  take  a mean  between 
two  estimations — one  slightly  under  and  the  other  slightly  over. 


Aiming  point. 


D O’ 


12 


IV.  RANGE  FINDING. 


22.  If,  in  range  finding,  a good  natural  aiming  point  can  not  be 
found,  a marker  or  picket  may  be  placed  to  give  an  aiming  point. 
The  distance  that  this  mark  should  be  placed  from  the  observer  de- 
pends on  the  distance  of  the  target.  Thus,  for  a range  of  1,000  yards, 
an  aiming  point  may  be  100  yards  away ; but  for  2,000  or  3,000  yards 
the  mark  should  certainly  not  be  nearer  than  200  yards.  It  is  a 
maxim  that  the  farther  the  aiming  point  the  more  accurate  is  the 
measurment  of  a range,  all  other  circumstances  being  equal. 

Should  an  observer  have  an  assistant,  he  may  send  him  100  yards 
to  the  left  to  serve  as  an  aiming  point,  while  he  himself  goes  the  same 
distance  to  the  right,  or  vice  versa;  thus  very  little  time  is  lost  in 
finding  an  aiming  point  for  a long  range. 

Problem  1. — To  Find  the  Range  or  an  Object  with  the  Aid  of 
Pickets  or  Men  Placed  as  Markers.  (Fig.  8.) 

23.  The  observer  near  X looks  in  the  first  prism  (90°)  and  sees 
the  image  of  O nearly  on  some  distant  point  or  marker,  D ; he  moves, 
as  before  explained,  until  the  reflection  of  O falls  true  on  D ; he.  plants 
a picket  at  this  point,  X,  and  retires  on  the  line  DX  prolonged 
toward  A;  when  he  thinks  he  has  gone  far  enough,  i.  e.,  nearly  one- 
fiftieth  of  the  estimated  range,  he  stops  to  observe  with  the  second 
prism,  say  at  O,  where  he  sees  in  the  second  prism  O on  his  right  of 
the  aiming  point  D.  This  shows  he  has  not  gone  far  enough,  so  he 
moves  on,  say  to  F;  he  now  sees  O reflected  to  his  left  of  D.  He 
soon  finds  A where  0 is  reflected  true  along  the  line  XD.  The  base 
AX  is  now  found,  and  it  is  one-fiftieth  of  the  range  XO. 


Problem  2. — To  Measure  Ranges  by  Using  Second  Prism  Only. 

24.  Use  the  point  from  which  the  range  is  to  be  measured  as  an 
aiming  point.  Hold  the  range  finder  by  method  (a)  or  (6)  (para- 
graph 9 above),  and  find,  by 
using  second  prism,  the  point  A, 
figure  2,  such  that  direction  of 
O seen  through  prism  coincides 

0 with  direction  of  D seen  di- 
rectly under  or  over  the  sec- 
ond prism.  Mark  point  A by 
a stake.  The  line  DA  should 
be  approximately  at  right  an- 
gles to  line  DO. 

25.  Proceed  along  AD  prolonged  beyond  Z>,  holding  the  range 
finder  by  method  (b)  or  (a)  (paragraph  9 above)  until  point  B is 
reached,  from  which  the  reflection  O seen  in  the  second  prism  coin- 
cides with  D or  A seen  under  or  over  the  prism. 

The  range  DO  is  25  times  the  base  BA. 


13 


Problem  3. — To  Calculate  a Long  Base  by  Measuring  a Short 
Base  one-fourth  its  Length. 

26.  If  the  base  XA  is  very  long,  or  the  ground  prevents  accurate 
use  of  the  tape,  the  base  should  be  measured  with  the  third  prism, 
thus:  The  points  X and  A (fig.  8)  being  marked  by  pickets,  the 
observer  attaches  the  measuring  tape  to  the  picket  A,  and  moves 
backward  on  the  line  OA , running  out  the  line  until  he  reaches  a 
point  B,  where  he  sees  in  the  third  prism  the  picket  X reflected  true 
on  0 along  the  line  BAO.  This  short  base  BA  is  one-quarter  of  the 
base  AX.  Every  yard  of  the  short  base,  therefore,  represents  200 
yards  of  range. 

27.  The  advantage  of  using  the  third  prism  to  measure  the  long 
base  AX  is  that  it  compensates  for  any  error  that  may  have  been 
made  in  determining  the  position  of  the  point  A , for  if  this  point 
be  erroneously  determined  so  that  AX  is  longer  than  one-fiftieth  of 
the  range,  then  the  angle  BAX , figure  8,  of  the  subsidiary  triangle 
ABX  will  be  enlarged,  while  the  other,  found  by  the  third  prism, 
remains  constant ; consequently,  the  base  AB  will  be  found  less  than 
one-fourth  of  the  erroneous  base  AX  and  more  nearly  the  true  range 
will  be  determined.  Conversely,  if  the  position  of  the  point  A be 
erroneously  determined  so  that  AX  is  less  than  one-fiftieth'  of  the 
range,  the  angle  BAX  will  be  lessened  and  the  base  AB  will  be 
found  more  than  one-fourth  of  AX 
and  more  nearly  the  true  range  will  be 
determined. 

Conversely,  if  the  original  base  AX 
be  made  less  than  one-fiftieth  of  the 
range,  the  angle  BAX  will  be  lessened 
and  the  base  AB  will  be  found  more 
than  one-quarter  of  AX,  or  nearly  the 
true  range. 

Problem  Jf. — To  Find  the  Range  of 
the  Object  without  the  Aid  of 
Pickets. 

28.  The  observer  at  A,  figure  3,  marks 
his  position,  and,  using  the  second 
prism,  reflects  the  target  O on  some 
distant  point,  D.  Then,  selecting  some 
intermediate  object  in  line  to  assist  him  in  marching  straight,  he 
moves  toward  D,  halting  occasionally  to  observe  with  the  first  prism, 
and  makes  sure  he  has  not  overshot  the  point  where,  as  at  X,  the 
target  O is  aligned  on  D.  When  he  has  found  X he  measures  the 
distance  from  X to  A and  thus  obtains  the  range. 


D 


14 


Problem  o. — To  Detekmine  the  Distance  between  any  Two 
Points  without  occupying  them. 

29.  The  observer  being  at  X.  figure  4.  to  ascertain  the  distance 
between  A and  B , he  sets  up  a picket  at  X.  and  from  it,  as  a fixed 
point,  with  an  aiming  point,  F,  at  right  angles  to  BX , lays  off  and 


3 


E 

Fig.  4. 


marks  the  extremity  of  the  base  XB  as  though  he  were  taking  the 
range  XB.  (Problem  1.) 

Again,  from  X,  with  an  aiming  point  X,  he  lays  off  and  marks  the 
base  XC  required  to  find  the  range  XA.  The  distance  between  the 
two  extremities  C and  I)  will  be  one-fiftieth  of  the  distance  between 
A and  B. 

War  Department, 

# 

Office  of  the  Chief  of  Ordnance, 

Washington,  September  2,  1911. 

Revised,  March  1,  1906. 

Revised  February  1,  1909. 

Revised  September  2,  1911. 

Form  No.  1908. 

Ed.  June  13-17 — 1,000. 

9847 /188-Encs.  1 and  2. 

O 


14 


Problem  5. — To  Determine  the  Distance  between  any  Two 
Points  without  occupying  them. 

29.  The  observer  being  at  X,  figure  4,  to  ascertain  the  distance 
between  A and  B , he  sets  up  a picket  at  X,  and  from  it,  as  a fixed 
point,  with  an  aiming  point,  F , at  right  angles  to  BX , lays  off  and 


B 


marks  the  extremity  of  the  base  XD  as  though  he  were  taking  the 
range  XB.  (Problem  1.) 

Again,  from  X,  with  an  aiming  point  E , he  lays  off  and  marks  the 
base  XG  required  to  find  the  range  XA.  The  distance  between  the 
two  extremities  G and  D will  be  one-fiftieth  of  the  distance  between 
A and  B. 

War  Department, 

Office  of  the  Chief  of  Ordnance, 

Washington , September  2,  1911. 

Revised,  March  1,  1906. 

Revised  February  1,  1909. 

Revised  September  2,  1911. 

Form  No.  1908. 

Ed.  June  13-17—1,000. 

9847/188— Encs.  1 and  2. 

O 


104720.  (Face  page  14.)  No.  1 


104720 — 17.  (Face  page  14.)  No.  2 Fig  8. 


